This invention relates to apparatus for measuring electrical operating conditions of a circuit and, more particularly, to apparatus for measuring electrical operating conditions of a circuit through alternating current (ac) probing. In the context of this invention, ac probing means applying ac signals to a circuit and observing the circuit's response to the applied ac signals.
Measurement of an electrical operating condition in a circuit can be made directly or indirectly. A direct measurement of current, for example, can be made by inserting a current meter in series with the line and by observing the deflection of the meter. An indirect measurement of current, for example, can be made by placing a small resistor in series with a line and by measuring the voltage developed across the resistor. Direct and indirect voltage measurements can be accomplished by analogous techniques.
Although the above measurement methods permit good measuring accuracy, they provide poor isolation between the measured circuit and the measuring circuit. Therefore, these measuring methods are poorly suited for circuit applications where the operating condition measured is subject to potentially destructive large voltage transients and where, therefore, good isolation between the measuring and measured circuits is desired.
One application where such isolation is desired is in the telephone network, where customer lines are sensed for current flow which indicates that a phone is "off-hook." For such a telephone application, C. F. Ault in U.S. Pat. No. 3,889,069, issued June 10, 1975, discloses a circuit which can detect current and which can simultaneously provide isolation to protect the measuring circuit from spurious high voltages (e.g., lightning induced voltages). The Ault circuit comprises a magnetic element having a winding to which the tested line is connected, an interrogate winding for developing pulses and a sense winding. When no current flows through the tested line, a pulse developed by the interrogate winding is inductively coupled to the sense winding and detected therein. When a current does flow through the tested line, the magnetic element becomes saturated by the current flowing through the winding to which the tested line is connected and the interrogate winding pulses to not, therefore, couple to the sense winding.
The desired isolation is indeed achieved by the Ault circuit. However, the circuit is expensive, cumbersome and lacks the ability to accurately indicate the current's value.
It is an object of this invention, therefore, to provide a circuit for measuring a line's signal while maintaining sufficient isolation therefrom to protect the measuring circuit from potentially destructive spurious voltages appearing on the measuring line.